The preneoplastic mammary gland and mammary tumors from different strains of mice were examined by electron microscopy to study virus-cell interaction and in vivo transformation. The role of virus (MTV) in the transformation of preneoplastic cells could not be established due to the simultaneous observation of virus particles and transformation of mammary gland cells. However, it was significantly observed that in GR strain mouse mammary tumors the virus is released primarily by the secretory process, whereas in DD plaques and C3H-Avy nodules the virus was released by the budding process.